In the field of continuously variable transmissions it is common for the power coming from the drive to be apportioned into multiple power paths in the transmission. In such a case at least one power path is designed with a variable fraction in which the transmission ratio between the input and the output can be adjusted variably.
In a power-split continuously variable transmission, the power is divided in the transmission, so that one fraction is directed via a path with a variable transmission ratio. The other fraction is usually directed mechanically parallel to the variable path. Subsequently the power paths are again summed via a connected gear unit. The power splitting is necessary due to the technical design. Two concepts for this are conventional in this field of technology.
The variable path is usually designed in the form of a bevel gear transmission (continuously variable transmission or “CVT”). Two shafts with respective opposing bevel gear pairs are provided. A push belt as a transmission means runs between the two pairs of bevel gears. It is used to transmit power from one pair of bevel gears to the other. The bevel gear pairs are each displaceable relative to one another in the axial direction, so that the contact length of the transmission means with the bevel gear pairs is adjusted according to the spacing of the bevel gear pairs, and the radius is simultaneously varied. By targeted control of the distances, the transmission ratio between the two shafts can be selected in a wide range.
Due to the design, the transmission means transmits the power to the bevel gears by means of frictional contact. The frictional contact and the associated power transmission are subject to fluctuations that are caused by raising and lowering the power. This results in fluctuations of the frictional force between the transmission means and the bevel gear pairs. Consequently, a relative movement arising from the drop of power transmission results.
The relative movement is to be avoided. This is accomplished by an appropriate control of the pressing force of the bevel gear pairs and in design terms by power-splitting so that only a part of the drive power is directed via the CVT. The remaining fraction is summed up again mechanically in the powertrain downstream of the CVT and directed to the output.
If this is used in high-powered drives, only a certain fraction of the power is directed via the CVT. The mechanical fraction requires a solid construction in order to direct the power past the variable path by means of classical gear units, normally spur gears or helical gears.
Another variant is the design of the variable transmission path in the form of a hydraulic transmission. In this case, the variable power fraction is converted by means of a hydraulic pump into hydraulic energy. By controlling the hydraulic pump and the hydraulic motor, the efficiency of the conversion of mechanical energy to hydraulic energy can be controlled in a wide range.
Due to the design, pressure losses occur within the hydraulic circuit, which lead to losses of power. Similarly to the CVT transmission, the hydraulics also have a hydraulic slippage, which is caused by a temperature increase of the hydraulic fluid. Therefore the controller for hydromechanical transmission systems is regulated and designed such that overheating of the hydraulic fluid is avoided.
This leads to a complex structure of transmissions for a variable transmission ratio, which have numerous components. The mechanical power path of such transmissions are adapted to the power limits of the variable transmission path. A considerable size may result, depending on the power level. Material costs rise because of the size and the associated weight of the transmission units. If used in a vehicle, this leads to an increase of weight and increased consumption.